Saw this thread and thought I'd pop in with my experience. I'm on the Explorer forum but pop in here once in a while. I had a 2017 Explorer that was my first experience with that model. When the 2018 1200 appeared I sold the 2017. But had the same problem mentioned here, a stiffer throttle spring that after an hour or so was fatiguing.
I solved the problem by carefully taking apart the throttle housing apart, figured out the spring attachment points and released a little of the tension on the internal return spring. I will warn you this is not for the faint of heart. It's full of grease in there, and lining up the parts is tricky, it took me several tries to get things right. If not done properly the bike won't run. Almost thought I was going to have to buy a new assembly but eventually got it and now have a lighter pull.
As an aside, there is also a spring mounted on the outside of the throttle housing that you push against to turn off the cruise. Triumph also made this spring stiffer for 2018. More of an annoyance really. Anyway, I shortened this spring a little so I don't have as far to turn.